The command reference does define this. The [url-server ] command has a "timeout" keyword that specifies how long the Pix waits before using the second server. It is a "backup" scenario rather than round-robin approach. Cisco routers can do load-balacing for this using IOS SLB. If the software is on Win2k you can use NLB to load-balance the requests, provided the servers are on the same subnet. Of course, you can always you an external load-balancing device.